This invention relates generally to transaxles including a hydrostatic transmission (xe2x80x9cHSTxe2x80x9d) commonly used with riding lawn mowers and similar small tractors. Such tractors generally use an engine having a vertical output shaft which is connected to the transaxle via a conventional belt and pulley system. A standard HST for such a transaxle includes a hydraulic pump, which is driven by the engine output shaft, and a hydraulic motor, both of which are usually mounted on a center section. Rotation of the pump by an input shaft creates an axial motion of the pump pistons. The oil pressure created by this axial motion is channelled via porting to the hydraulic motor, where it is received by the motor pistons, and the axial motion of these pistons against a thrust bearing causes the motor to rotate. The hydraulic motor in turn has an output shaft which drives the vehicle axles through differential gearing.
Among the advantages of transaxles with hydrostatic transmissions are the reduction of the number of parts and in the size of the unit, and, in some instances, the elimination of mechanical gears. As is known in the art, the use of a transaxle having a hydrostatic transmission enables the manufacturer to include all necessary elements in one unit, whereby the transaxle is easily incorporated into the tractor design, as it requires only the addition of a belt to connect it to the motor and a control lever for changing speed and direction. While the basic principles of transaxles with an HST are well known in the prior art, there are several disadvantages of present transaxles with HST designs. These disadvantages, and the present invention""s means for overcoming them, are set forth herein.
A major problem with some prior transaxle designs is that the transmission is too large and too expensive to be used with the smaller tractors where it would be most effective. An attempt to solve this problem is shown in Okada, U.S. Pat. Nos. 4,914,907 and 4,932,209. The Okada ""209 patent discloses a first mechanical deceleration means, namely the gear on the motor shaft and countershaft within the axle housing, and a second mechanical deceleration means in the differential. The gearing in the deceleration means eventually transmits power to the differential gears, which are then used to drive the output axle. However, these mechanical deceleration units add unnecessary weight and expense to the unit. An object of the present invention is to provide an transaxle design which does not require such additional mechanical deceleration means.
Another variation on the standard transaxle with HST design is shown in Thoma, U.S. Pat. No. 4,979,583. This patent teaches the segregation of the hydraulic units from the remaining portions of the transaxle through the use of separate segregated cavities to house each. In addition, the pump and motor in the Thoma design are mounted back-to-back, so that the input and output shafts have the same orientation. Thus additional gear units are required to re-orient the rotation of the output shaft so that it is parallel to the ultimate drive axle. Further gears then drive a differential which rotates the drive axle. This additional gearing adds weight to the unit and expense to the manufacturing process.
Thus, the Okada and Thoma designs present problems from the standpoint of manufacturing a small, economical transaxle including an HST which is easily adaptable to different size tractors or axle configuration. Okada requires multiple gearing and Thoma requires a housing having segregated cavities. The present invention is designed to overcome these and other problems in the prior art by providing a compact, economical transaxle with HST which substantially reduces the number of moving parts previously required.
The present invention, sometimes referred to generally as a xe2x80x9ctransaxle,xe2x80x9d includes a split-axle housing which encases an HST. The HST includes a pump and a motor whose orientation to one another may be varied according to the space requirements dictated by the size and configuration of the vehicle. This transaxle also includes a novel hydraulic reduction means, an improved differential, a longer lasting, more effective means of preventing oil leakage from the axle shafts in the housing, a center section supporting the output drive shaft, an improved means for hydraulically bypassing the HST and a unique check valve arrangement. Each of the specific novel improvements are combined to provide a transaxle which is compact, reliable and economical to manufacture. These and other objects and improvements of this invention will be set forth in more detail herein.
One object of this invention is to provide an improved transaxle wherein the center section of the HST, on which the pump and motor are mounted, also serves as the bearing support of the output drive shaft. In the prior art, for example, Okada U.S. Pat. No. 4,932,209, one end of the gear drive arrangement is supported in the center section, but the other end is supported by the upper and lower axle housing casings.
The advantage of the present invention""s arrangement is that it eliminates the need for an additional bearing support, thus reducing the costs and assembly time required. It also eliminates the tolerance concerns for aligning the bearing supports for the output drive shaft.
A further object of this invention is to provide a transaxle that may use multiple mechanical reduction units, but requires only a single such unit because a portion of the overall reduction is provided hydrostatically. The prior art generally requires dual or multiple mechanical reduction units in conjunction with the hydraulic unit. For example, as set forth above, U.S. Pat. No. 4,932,209 requires the use of two separate mechanical reduction units, including a separate counter-shaft between the hydraulic motor and the differential used to drive the output axle.
The present invention makes this same reduction through the hydraulics itself by the use of a motor which is larger in displacement than the pump. This eliminates the need for any secondary mechanical reduction units, thereby reducing sources of possible mechanical failure. The single reduction arrangement reduces the number of necessary components and the size of the transmission, and it eliminates the need for an additional support shaft or jack shafts, thus resulting in a smaller, simpler and less expensive transaxle. In a heavy duty application, the prior art often used two sets of mechanical reduction units to handle the necessary reduction. In such instances, the present invention""s hydraulic reduction can eliminate the need for such multiple reduction units or could be used in conjunction with secondary units only.
A further object of this invention is to restrict the oil from having to extend to the outer axle support bearings, as is common in prior art models. The gearing and the hydrostatic transmission element of this invention are enclosed in a single chamber formed by an upper casing and a lower casing. The axle shafts extend through this chamber and are supported by separate bearing surfaces outside of the chamber.
In most of the prior art, the entire axle casing is filled with oil out to the outer axle bearings to provide lubrication to these bearings, in addition to the hydrostatic function of the oil in the pump and motor. However, after the outer axle bearings wear through use, the eccentricity or xe2x80x9cplayxe2x80x9d in the shaft may distort the oil seal at said outer bearings, allowing the leakage of oil out of the main chamber. Maintenance of a leak-free joint is critical to the function and appearance of such a transaxle with HST unit. The entire internal hydraulic parts of an HST should be covered with oil, as an insufficient amount of oil in the main transmission cavity will cause foaming of the oil, damaging the hydraulic structures. Excessive oil leakage is a serious problem as it will hamper the ability of the HST to operate and cause damage to the internal workings of the HST. Oil leakage also presents an aesthetic problem for manufacturers of transaxles, as customers are usually quite disturbed by the presence of oil leaks and the accompanying oil stains. Thus, the reduction or elimination of oil leakage is critical for the continued success of transaxle sales.
In the prior art, maintenance of such a leak-free joint at the outer bearings requires the use of extra bolts and sealant, which add additional weight and cost to the unit. An additional problem with prior art designs is that such wear in the outer axle bearings can also cause contamination of the oil due to the presence of xe2x80x9cshavingsxe2x80x9d and other detritus from the worn bearings.
Although such construction could be used with the other novel elements of the present invention, to solve these problems of leakage and potential oil contamination at minimum cost, the present invention also presents a unique means of restricting the oil to those portions of the transaxle where it is needed to lubricate the differential and to work the pump and motor of the HST. Thus, chambers separate from the main chamber enclosing the HST and differential surround the majority of each axle shaft. Therefore, the oil does not extend throughout the entire casing or to the outer axle bearings, removing the potential problem of oil leaking from the casing. Separate grease pockets are used to lubricate these outer axle bearings, resulting in a much more durable seal and allowing for the use of a higher viscosity grease lubricate these outer axle bearings.
This improvement also allows for a reduction in the amount of oil needed to fill the transmission case, and, due to the reduced sealant area at the outer axle bearings, a reduction in the amount of sealant required. Due to the fact that the maintenance of a leak-free joint at the outer axle bearings is not required, this invention also allows for reduced manufacturing tolerances, which reduces the manufacturing costs of the unit.
A further improvement is in the method used to place the transaxle into neutral gear to enable movement of the tractor without the motor running. A problem with the typical HST arrangement is that xe2x80x9cneutral gearxe2x80x9d does not exist, as it is merely a point where the hydraulic pressure in the pump goes to zero. However, at this point the oil remains in the transmission, preventing the vehicle from being rolled freely.
The prior art generally solves this problem by diverting the oil through a hydraulic valve from the pressure side to the vacuum side of the HST center section. The problem with such a design is that the hydraulic valve allows for the movement of only a limited amount of oil due to inherent design limitations, such as the diameter of the hydraulic value through which the oil is diverted. Furthermore, machining such a valve requires precise tolerances, thus increasing the manufacturing costs of the unit.
In the present invention, this problem is solved by providing a mechanism whereby the motor block is mechanically lifted from its running surface, thereby allowing the oil to bypass the vacuum-pressure circuit and to exit the case completely. This operates to enable the vehicle to freewheel more easily than is possible with the prior art hydraulic valve method.
Another object of the present invention is to provide an improved design of the motor and motor thrust bearing in a hydrostatic transmission, whereby the motor shaft does not extend through the motor thrust bearing, and thus the bearing is fully supported and does not require an intermediate support plate, as is used on prior art models.
For example, U.S. Pat. No. 4,953,426 to Johnson teaches a thrust bearing having a motor shaft extending through its center section. As in the present invention, the thrust bearing in Patent ""426 is supported by one section of the housing. However, because the ""426 thrust bearing has the motor shaft extending through its center, it is not solely supported by the housing, but rather is supported by two xe2x80x9cfingersxe2x80x9d on either side of the thrust bearing. To support the thrust bearing against the hydraulic forces applied by the motor pistons, an additional structurally significant piece is required to support between these fingers.
In the present invention, the thrust bearing is fully supported by the housing part into which it is inserted, thus eliminating the need for an additional structural member. This results in an assembly that is simpler and less expensive to manufacture.
A further object of this invention is an improved differential gear assembly. In the prior art, differential assemblies generally require a cross-shaft to support the planet bevel gears. The arrangement of the present invention eliminates the need to use such a cross-shaft by providing a simple end cap axle support and bevel and planet gear entrapment.
A further novel feature of this invention is in the placement of the brake portion in the housing. Disk brakes are known in the art, and generally consist of a series of disks or plates, mounted on or about a rotating shaft, with at least some of the disks or plates rotating with the shaft. Such brakes generally have a brake arm or level which is moved to activate the braking feature by a means for transmitting the movement of the brake arm to the series of disks, causing the stationary disks to be pressed against the rotating disks, thus braking this rotating shaft through friction. This means for transmitting the movement of the brake arm to the disks generally consists of rods or shafts, and, in the prior art, these rods or shafts were mounted in a housing which is separate from the housing containing the HST. In the present invention, the brake rods are mounted directly into the HST housing through half-round sections formed into each of the mating housing sections, thus eliminating the need for this separate housing and reducing the manufacturing costs of the products.
An additional novel feature of this invention is the design of the check valve for the center section. Prior art check valve designs generally use hardened steel balls working against a steel or cast iron seat. To minimize the overall weight of the transaxle unit, however, the center section of the present invention is preferably made of cast aluminum, which is not strong enough to function as such a valve seat and to withstand the wear from such a check valve operation. This problem is solved by the use of a steel insert in the center section to support the steel balls.
To create a seal at such a location, it is known to use a machined surface on both the seat and the insert, so that a standard O-ring seal could be used. However, use of such a sealing means would require additional machining steps on the seat and insert, adding to the overall manufacturing costs of the unit.
To overcome these problems in the prior art, the present invention calls for the use of a powdered metal plate which acts as both the check valve seat and as the seal. The sealing functions of the plate are created through the use of a raised surface on the plate, which is pressed into the lower strength aluminum to form a seal. This design has the advantage of being simple and inexpensive to manufacture, while maintaining the advantage of a light overall weight.
It is a further object of this invention to provide an improved hydrostatic transmission wherein the pump and the motor of the HST need not be orientated at a 90-degree angle to one another as required by the prior art. In the present invention, the 90-degree orientation is the preferred embodiment. However, an orientation other than 90-degrees can be achieved by use of a helical gear between the output drive shaft and the differential.
Further explanation and details of the above objects of this invention, as well as other benefits and advantages of this invention, will be set forth in the following sections.